1. Field of the Disclosure
Embodiments disclosed herein relate generally to fixed cutter drill bits having a partial connection of the blades at the bit center area. More specifically, embodiments disclosed herein relate to fixed cutter drill bits having a partial connection that partially connects primary blades of the fixed cutter drill bit.
2. Background Art
Fixed cutter bits, such as PDC drill bits, are commonly used in the oil and gas industry to drill well bores. One example of a conventional drilling system for drilling boreholes in subsurface earth formations is shown in FIG. 1. This drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14. Connected to the end of the drill string 12 is a fixed cutter drill bit 20.
As shown in FIG. 2, a fixed cutter drill bit 20 typically includes a bit body 22 having an externally threaded connection at one end 24, and a plurality of blades 26 extending from the other end of bit body 22 and forming the cutting surface of the bit 20. A plurality of cutters 28 are attached to each of the blades 26 and extend from the blades to cut through earth formations when the bit 20 is rotated during drilling. The cutters 28 deform the earth formation by scraping and shearing. The cutters 28 may be, for example, polycrystalline diamond compacts, or any other cutting elements of materials hard and strong enough to deform or cut through the formation. Hardfacing (not shown) may also be applied to the cutters 28 and other portions of the bit 20 to reduce wear on the bit 20 and to increase the life of the bit 20 as the bit 20 cuts through earth formations.
Fixed cutter bits used to drill oil and gas well bores are traditionally manufactured to include a plurality of blades 26 formed on the surface of the drill bit 20. Typically, the blades 26 have been manufactured to extend from an outer gage 27 to a center 29 of the bit 20. However, the center 29 of the bit 30 is generally left open such that each blade 26 forms an independent portion of the bit 20. Fixed cutter bits 20 with blades 26 that are disconnected are generally referred to in the art as separated blade fixed cutter bits.
Referring to FIG. 3, a cross-sectional view of a separated blade fixed cutter bit 30 is shown. Separated blade fixed cutter bit 30 is illustrated having two primary blades 31, with a plurality of cutters 32 disposed on each of primary blades 31. Primary blades 31 have a blade height 33 defined as the top of the structure of the blade and a blade base 34 defining the intersection of each primary blade 31 with the structural base of the bit. As such, primary blades 31 may be defined as the protruding area between blade base 34 and blade height 33. Primary blades 31 extend from an outer gage 37 to a bit center area 38 (illustrated here as an axis extending through the geometric center of the bit). However, as illustrated, primary blades 31 do not connect at bit center area 38. Rather, primary blades 31 remain separated and an open area 39, defined as the area between primary blades 31, is formed.
During drilling, it was found that that center of separated blade fixed cutter bits were structurally weak and prone to cracking after repair. Separated blade fixed cutter bits would often form cracks that run across the bit center area between the nozzles, thereby causing the bits to be unrepairable.
To overcome the problems associated with separated blade fixed cutter bits, fixed cutter drill bit designs were changed such that major blades were connected at the bit center area. Because the blades were connected, and the amount of matrix or steel at the bit center area was thereby increased, it was believed that the structural integrity of the bits would be increased.
Referring to FIG. 4, a cross-section view of a fully connected fixed cutter bit 40 is shown. Fully connected fixed cutter bit 40 is illustrated having two primary blades 41, with a plurality of cutters 42 disposed on each of primary blades 41. Like the separated blade fixed cutter bit 30 (FIG. 3) discussed above, primary blades 41 of fully connected fixed cutter bit 40 have a blade height 43 defined as the top of the structure of the blade and a blade base 44 defining the intersection of each primary blade 41 with the structural base of the bit. Primary blades 41 extend from an outer gage 47 to a bit center area 48. However, unlike separated blade fixed cutter bit 30 (FIG. 3), primary blades 41 are connected at the bit center area 48. As such, blade height 43 extends between primary blades 41, thereby forming a continuous structural element.
When used in drilling, the fully connected primary blades 41 did in fact stop some of the cracking, typically stopping cracks from running across bit center area 48 and between nozzle bores (not shown). However, fully connected primary blades 41 did not stop cracks from forming. Instead of forming across bit center area 48, the cracks shifted to forming behind first, second, and third cutters 49A, 49B, and 49C. Finite element analysis revealed that the fully connected bit designs resulted in thermal residual stress. Furthermore, it was determined that behind first, second, and third cutters 49A, 49B, and 49C, thermal stress (i.e., stress from thermal shock during brazing) and mechanical stress (i.e., stress from drilling), combined to produce cracks that formed more often, earlier, and with less options available for repair.
Accordingly, there exists a continuing need for a fixed cutter bit design that may provide increased structural integrity and resist cracking during drilling.